Vapour phase chemical infiltration process for densifying porous substrates disposed in annular stacks

ABSTRACT

A chemical vapor infiltration method for densifying porous substrates disposed in annular stacks. The substrates to be densified (12) are loaded inside a reaction chamber (11) of an infiltration oven, being disposed in at least one annular stack (30) defining an interior passage (31) with spaces (33) being formed between the substrates. The gas admitted into the reaction chamber is channelled on leaving a preheating zone (18) towards one of the two volumes constituted by the inside and the outside of the, or each, stack of substrates, and preferably towards the smaller volume. This volume (31) is closed at its opposite end so that the gas flows from admission into the chamber to its exhaust from the chamber from the inside to the outside of the, or each, stack, or vice versa, with the gas passing through the spaces between the substrates and diffusing into them.

The present invention relates to a chemical vapor infiltration methodfor densifying porous substrates disposed in annular stacks, i.e.substrates that are substantially bodies of revolution with a centralopening or passage and which are disposed in at least one stack definingan interior passage formed by the central openings of the substrates, orsubstrates that are not necessarily annular in shape, but which aredisposed to form a stack with an interior passage defined by the stackedsubstrates.

BACKGROUND OF THE INVENTION

The field of application of the invention lies in particular inmanufacturing composite material parts comprising a porous substrate or"preform" densified by a matrix.

To manufacture composite material parts, in particular thermostructuralcomposite material parts constituted by a refractory fiber preform (e.g.carbon or ceramic fibers) densified by a refractory matrix (e.g. carbonor ceramic), it is common practice to use chemical vapor infiltrationmethods. Examples of such parts are carbon--carbon (C--C) compositenozzles for thrusters, or C--C composite brake disks, in particular forairplane brakes.

Densifying porous substrates by chemical vapor infiltration consists inplacing the substrates in a reaction chamber of an infiltrationinstallation by means of support tooling, and in admitting into thechamber a gas having one or more components constituted by precursorsfor the material that is to be deposited within the substrates for thepurpose of densifying them. Infiltration conditions, in particular gascomposition and flow rate, and also temperature and pressure inside thechamber are selected to enable the gas to diffuse within the accessibleinternal pores of the substrates so that the desired material isdeposited therein by a component of the gas decomposing or by a reactionbetween a plurality of the components thereof.

The conditions required for chemical vapor infiltration of pyrolyticcarbon or "pyrocarbon" have been known for a long time to the personskilled in the art. The precursor for carbon is an alkane, an alkyl, oran alkene, generally propane, methane, or a mixture thereof.Infiltration is performed at a temperature of about 1000° C. at apressure of about 1 kPa, for example. The infiltration conditionsrequired for chemical vapor infiltration of materials other than carbon,in particular ceramic materials, are also well known. On this topic,reference may be made in particular to document FR-A-2 401 888.

In an industrial installation for chemical vapor infiltration, it isusual to load the reaction chamber with a plurality of substrates orpreforms to be densified simultaneously, by using support toolingcomprising, in particular, trays and spacers. When the preforms areannular, they may be stacked in a longitudinal direction of the reactionchamber. The gas containing the precursor(s) of the material to bedeposited within the preforms is admitted at one longitudinal end of thechamber, while the residual gas is evacuated from the opposite end whereit is extracted by pumping means. Means are generally provided topreheat the gas before it reaches the preforms to be densified, e.g.means in the form of perforated preheating plates through which the gaspasses on being admitted into the reaction chamber.

A real difficulty encountered with known chemical vapor infiltrationmethods is to ensure that the microstructure of the material depositedwithin the substrates is constant. In the particular case of compositematerial parts, the expected properties of said parts require themicrostructure of the matrix to be constant and of the kind desired.Thus, in the example of infiltrating pyrolytic carbon or "pyrocarbon",variations in infiltration conditions, even very small variations, canlead to changes in the microstructure of the pyrocarbon. Unfortunatelypyrocarbons of the smooth laminar type, of the rough laminar type, andof the isotropic type have properties that are quite distinct. Forexample, if it is desired to obtain a graphitable pyrocarbon matrix byheat treatment, it is preferable to obtain a rough laminar typemicrostructure. In practice, in spite of the care given to controllinginfiltration conditions, changes are observed in the microstructure ofthe pyrocarbon deposited within preforms, in particular within thepreforms that are furthest from the access for the gas into the chamber.Such irregular microstructure has sometimes gone as far as forming sootand as forming undesirable dendritic growths in the reaction chamber.

To solve that problem, attempts have been made to significantly increasethe flow rate of the gas admitted into the chamber, such that similargas is presented to all of the preforms in the load. However it is thennecessary to provide a more powerful pumping device, which is thereforemore expensive, and more gas is consumed. In addition, the effectivenessof the preheating is decreased if the gas passes more quickly throughthe preheater plates. To bring the gas to the desired temperature notlater than its first contact with a preform to be densified, it isnecessary to increase the number of preheating plates, but that isdetrimental to the working volume available inside the chamber, and thusto the overall throughput of the installation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method enabling theabove-mentioned drawbacks to be avoided, i.e. a chemical vaporinfiltration method that makes it possible to guarantee constancy of themicrostructure deposited within the densified substrates, and to do sowithout requiring an increase in the flow rate of the gas or arestriction on the loadable volume both of which are harmful in terms ofthe cost and the throughput of the installation.

This object is achieved by a chemical vapor infiltration method fordensifying porous substrates by depositing material within thesubstrates, the method comprising: loading substrates to be densifiedinto a reaction chamber of an infiltration oven, the substrates beingdisposed in at least one annular or hollow stack which extends in alongitudinal direction of the chamber and which defines an interiorpassage with spaces being formed between the substrates; admitting a gascontaining at least a precursor of the material to be deposited into thereaction chamber in the vicinity of a first longitudinal end thereof;and exhausting the residual gas via an outlet situated in the vicinityof the longitudinal end of the reaction chamber opposite from the firstlongitudinal end;

in which method the gas admitted into the reaction chamber is channeledtowards one of the two volumes constituted by the inside and the outsideof the stack(s) of substrates at the end thereof closer to the firstlongitudinal end of the chamber; and the volume into which the gas ischanneled is closed at its end further from the first longitudinal endof the chamber; such that between admission into the chamber and exhaustfrom the chamber, the gas flows from the inside towards the outside ofthe, or each, stack, or vice versa, with the gas passing through thespaces between the substrates and diffusing into them.

Advantageously, when the gas admitted into the chamber is preheated bypassing through a preheating zone situated at the first end of thechamber, the channeling of the gas towards the inside or the outside ofthe, or each, stack is performed at the outlet from the preheating zone.

This method provides a clear improvement in infiltration conditions forthe purpose of achieving the desired object, in particular when comparedwith known methods in which the gas is admitted uniformly into thereaction chamber, i.e. when the chamber contains one or more stacks ofsubstrates, with the gas being admitted simultaneously to the inside andto the outside of the, or each, stack.

A very important advantage of the method of the invention is that theretention time of the gas in the reaction chamber can be considerablydecreased, without changing the admission flow rate. The gas is admittedonly into the volume constituted either by the empty space formed by thecentral passage(s) of the stack(s) of substrates, or by the empty spacearound the stack(s) of substrates. This volume is very considerablysmaller that the total volume of the reaction chamber that is notoccupied by the load (the substrates and the support tooling), i.e. thecombined volume of said empty spaces, such that a given quantity of gasflows much faster. In an industrial installation for infiltratingpyrocarbon in vapor form, the method of the invention has made it easyto limit the retention time to a value that is no greater than 1 second.Reducing retention time avoids excessive maturing and spoiling of thegas which could have the effect of changing the microstructure of thedeposited material.

In addition, since the option is provided of greatly reducing retentiontime for a given flow rate, it is possible, insofar as the resultsobtained remain acceptable, to make do with a smaller reduction inretention time, or indeed to leave it unchanged, in which case the gasflow rate is reduced. Reducing the flow rate provides a saving in gasconsumption. It also serves to reduce the bulk of the preheater means,and thus to increase the working volume of the oven, and it makes itpossible to use smaller pumping means.

In order to optimize the reduction in retention time and/or the flowrate of the gas, it is preferable for the admitted gas to be channelledtowards the smaller of the two volumes constituted by the inside and theoutside of the stack(s) of substrates.

An additional advantage of the method lies in the fact that by requiringthe gas to flow from the inside towards the outside of the, or each,pile of substrates, or vice versa, it is ensured that the surfaces ofthe substrates from which diffusion takes place towards the insides ofthe substrates are immersed in gas which is continuously renewed. Whenthe gas is admitted into one end of both of the volumes as constitutedby the inside and by the outside of the stack(s) of substrates, and whensaid volumes are not closed at the other end, then flow takes placepreferentially in the longitudinal direction. Continuous renewal of thegas can then no longer be guaranteed in the spaces between thesubstrates, unless those spaces are made to be large enough.Unfortunately, stagnation of the gas in the spaces between thesubstrates means that the retention time becomes large, therebydegrading the microstructure of the deposited material. If thesubstrates are spaced apart from one another by a distance which islarge enough to enhance the flow of gas between them, then that is tothe detriment of the occupancy rate of substrates in the infiltrationoven.

With the method of the invention, flow necessarily takes place incontinuous manner in the spaces between substrates, from the insidetowards the outside of the, or each, stack, or vice versa. It is thenpossible for the spaces formed between the substrates in a stack to benarrow, and merely sufficient to ensure pressure balancing between theinside and the outside of the stack. This makes it possible to optimizethe occupancy rate of the oven by the substrates.

It can be desirable to maintain a constant flow rate of the gas in thelongitudinal direction within the inside or outside volume towards whichit is channelled, in spite of the headlosses caused by lateral leaksinto the spaces between the substrates and in spite of diffusion intothe substrates. To this end, it is possible to place in said volume atleast one compensation element which extends in the longitudinaldirection with a cross-section that increases in the flow direction ofthe gas.

The method of the invention is advantageously used to densify annularpreforms for brake disks. The preforms may be placed in a stack or in aplurality of parallel stacks in the longitudinal direction of thereaction chamber. The gas admitted is then preferably channelled towardsthe insides of the stacks of preforms.

The method of the invention can also be used for densifying otherpreforms that are annular or substantially annular, in particularpreforms for the diverging parts of thruster nozzles. The preforms arethen placed one above another, preferably ensuring that each preform ispartially engaged in another. Since the internal passage through eachpreform has a relatively large diameter, the admitted gas is thenchanneled towards the outside of the, or each, stack of preforms, sincethat normally provides a smaller volume than does the inside.

The method of the invention can also be used to densify substrates thatare not necessarily annular, i.e. that are not necessarily bodies ofrevolution with a central opening or passage. Under such circumstances,the stack of substrates is made in such a manner as to form at least oneannular or hollow stack with an interior passage surrounded by thestacked substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the method of the invention are described below byway of non-limiting indication.

Reference is made to the accompanying drawings, in which:

FIG. 1 is a diagram showing how a reaction chamber of a chemical vaporinfiltration installation is filled in a known method;

FIG. 2 is a diagram showing an implementation of the method of theinvention for densifying annular preforms for brake disks;

FIG. 3 is a diagram of another implementation of the method of theinvention for densifying preforms for the diverging parts of thrusternozzles; and

FIG. 4 is a diagram of another way of loading substrates suitable forenabling the method of the invention to be implemented.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram of a reaction chamber 1 of a chemical vaporinfiltration installation. The chamber 1 is generally cylindrical inshape about a vertical axis. Annular fiber preforms 2, e.g. formanufacturing airplane brake disks made of carbon-carbon composite, areloaded into the chamber 1 in a configuration that is conventional in thestate of the art.

The preforms are disposed in a plurality of stacks extending in thevertical longitudinal direction of the chamber 1 (only two stacks areshown). The stacks are supported by tooling comprising bottom andintermediate loading trays 5a and 5b having holes 5 to allow gas to passthrough, together with spacers 5c between the trays. The entire assemblyrests on the bottom 1a of the chamber.

In order to densify the preforms 2, a gas containing a precursor ofcarbon, such as propane mixed with methane or natural gas, is injectedinto the chamber 1. In the example shown, the gas is conveyed by aplurality of ducts 6 which open out into the bottom portion of thechamber at locations that are more or less regularly spaced apart. Theresidual gases are extracted from the top portion of the chamber viaducts 7.

In the bottom portion of the chamber, the gas passes through apreheating zone 8 containing perforated preheating plates, prior toreaching the bottom loading tray 5a. The preheating plates are situatedin the chamber 1a and they are always close to the temperature whichobtains inside the chamber, thereby enabling the gas to be preheatedeffectively.

The inside of the chamber is heated by a graphite susceptor 9 forming aheater core that is electro-magnetically coupled with an inductor (notshown). The susceptor 9 defines the inside volume of the vertical axischamber whose bottom 1a has the ducts 6 passing therethrough and whosecover 1b has the ducts 7 passing therethrough. The bottom and the coverare also made of graphite as are the various plates, trays, and spacerscontained inside the chamber.

In well-known manner, the fiber preforms 2 are densified by depositingpyrolytic carbon therein as produced by decomposing the precursor whichis contained in the gas that diffuses inside the accessible internalpores of the preforms. In the chamber 1, between the preheating zone 8and the outlet ducts 7, the gas flows by passing inside and outside thestacks of substrates 2. In order to allow the gas to access the faces ofthe preforms 2, they are held apart from one another in each of thestacks, by means of spacers 3 which leave spaces 4 between thesubstrates.

In order to densify annular preforms using a method of the invention,the reaction chamber is loaded differently, as shown in FIG. 2.

As in the chamber shown in FIG. 1, the reaction chamber 11 iscylindrical in space about a vertical axis and is defined by a graphitesusceptor 19, a bottom 11a of graphite closing the bottom portion of thechamber, and a graphite cover 11b closing the top portion of thechamber.

In conventional manner, the infiltration installation includes aninductor (not shown) surrounding the susceptor 19. The inductor iscoupled to the susceptor 19 which acts as a heating core, for thepurpose of heating the chamber 11. Power supply to the inductor iscontrolled so as to maintain the temperature that obtains inside thechamber 11, as measured by means of a sensor (not shown) at the desiredvalue.

The preforms 12 are carbon fiber preforms for densifying by means of apyrolytic carbon matrix, e.g. preforms for airplane brake disks. Theyare made up of superposed carbon fiber plies bonded together byneedling. A method of making carbon fiber preforms constituted by pliesthat are stacked flat and then needled together is described, inparticular in document FR-A-2 584 106.

The gas containing one or more precursors for carbon is injected intothe chamber 11 via feed ducts 16 leading to the bottom end of thechamber through its bottom 11a. The gas giving rise to pyrolytic carbonis constituted, for example, by propane, a carbon precursor, and bynatural gas. The residual gas is extracted from the top portion of thechamber by means of exhaust ducts 17 passing through the cover 11b. Theexhaust ducts are connected to a pump device (not shown), enabling thedesired pressure to be established inside the chamber.

The gas penetrating into the chamber 11 is preheated by passing througha plurality of perforated preheating plates 20 which are spaced partfrom one anther and from the bottom 11a by spacers 21. The preheatingplates 20 and the spacers 21 are made of graphite. By passing throughthe plates 20, the gas is raised to a temperature close to that whichobtains inside the chamber 11.

The preheated gas then passes through a diffusing plate 22 which restson the bottom 11a via legs 23. The plate 22 has passages 22a at regularintervals for the purpose of distributing the gas in substantiallyuniform manner over the entire section of the chamber 11.

The preforms 12 are identical and they are disposed in vertical stacks20 on circular support trays 15a and 15b. These trays have holes 15 andthey are held spaced apart from one another by spacers 24. The supporttrays 15a and 15b and the spacers 24 are made of graphite, for example.The bottom support tray 15a stands on the diffuser plate 22 via blocks25 which keep it spaced apart therefrom. A circular perforated top plate26 can be placed above the load so as to make the temperature of theload uniform by screening the preforms situated in the top portion ofthe load against thermal radiation. The plate 26 rests on the topsupport tray via spacers 27. A plurality of stacks of preforms are builtup and are distributed more or less regularly over the surfaces of thetrays 15a and 15b (only two stacks are shown in FIG. 2). Each stack 30comprises a plurality of preforms 12 disposed one above another along acommon vertical axis and occupying the gap between two trays. Thepreforms stacked on the various trays 15a, 15b are vertically alignedwith the holes 15, which holes have diameters that are equal to orslightly greater than the inside diameters of the preforms 12. Thus, thestacks 30 of vertically-aligned preforms define respective centralpassages in the form of chimneys 31 constituted by the central openingsin the annular preforms 12 and the holes 15 in the trays. These passages31 are closed at their top ends by solid screens 32.

The preheated gas coming from the diffuser plate 22 is channelledtowards the volume constituted by the interior passages 31 of the stacks30. To this end, the blocks 25 between the diffuser plate 22 and thebottom support tray 15a are constituted by rings of diameter equal to orslightly greater than the diameter of the holes 15 and they are inalignment therewith so that the gas is directed exclusively into thepassages 31. The diffuser plate 22 is provided with perforations 22asolely in register with the passages 31.

Spacer elements 33 of small thickness are placed in each pile betweenthe preforms 12, or at least between groups of stacked-togetherpreforms. Similar spacer elements are also disposed between the supporttrays and the first preforms supported thereby, and also between thelast preforms in the stacks 30 and the screens 32. These spacer elements33 provide leakage passages 34 for the gas between the inside and theoutside of the preforms, allowing pressure to be balanced between thepassages 31 and the internal volume of the chamber 11, and enabling thegas to diffuse from the main plane faces of the preforms 12. In thisway, the gas coming from the preheating zone is channeled towards theinternal passages 31 of the stacks 30 and then flows from the insidetowards the outside of each stack 30 into the volume 36 of the chamberoutside the stacks 30, from which it is removed via the perforated plate26 and the outlet ducts 17.

In order to compensate the headloss due to these lateral leaks which arealso accompanied by the diffusion of the gas into the preforms, and forthe purpose of ensuring that the flow speed of the gas is substantiallyconstant along the passages 31, the circular flow section of thepassages can be decreased progressively in the flow direction of thegas, from the bottom towards the top. This decrease in flow section canbe obtained by inserting a central vertical tooling element 35 ofincreasing section in the gas flow direction inside each passage 31.This tooling element 35 (only one of which is shown in FIG. 2) is in theform of a "stalactite", e.g. being fixed to the underside of the screen32 situated at the top of the passage.

Compared with a disposition of the kind shown in FIG. 1, and for a givenflow rate of gas admission, a significant advantage of the method of theinvention is that the retention time of the gas in the chamber 11 isreduced, and constant renewal is guaranteed of the gas in which theoutside surfaces of the preforms are immersed.

The retention time in a reaction chamber of an industrial oven has beenmeasured. For a given admission flow rate, the retention time, measuredbetween the feed ducts 16 and the tops of the stacks of preforms was 0.4seconds (s) using a disposition of the kind shown in FIG. 2, whereas itwas 1.8 s with the disposition of FIG. 1. This reduction in retentiontime, due to the fact that the gas is channelled into a small volumeconstituted by the interior passages of the stacks, guarantees that themicrostructure of the pyrocarbon deposited over the full length of thestacks of preforms remains constant.

Also, by necessarily establishing flow between the inside and theoutside of each stack, continuous renewal of the gas is obtained in thespaces between the stacked preforms as provided by the spacers 34. Thesespaces can therefore be narrow, much less than 5 mm, e.g. having athickness lying in the range about 0.1 mm to about 5 mm, therebyenabling the occupancy rate of the oven to be optimized. In contrast, inthe disposition of FIG. 1, the gas flows preferentially in the verticaldirection inside and outside the stacks of preforms. To ensure thatsufficient flow exists along the spaces between the stacked preforms, itis necessary to provide spaces of relatively great width, to thedetriment of the occupancy rate of the oven. If such large gaps are notprovided, there is a risk of the gas stagnating in the spaces betweenthe preforms, and consequently, there is a risk of the microstructure ofthe pyrocarbon deposited by diffusion of said gas becoming degraded.

Also, since the retention time can be greatly reduced for constant flowrate when using the method of the invention, it is possible at constantretention time to reduce the flow rate correspondingly. Thus, if aconsiderable decrease in retention time is of no advantage in ensuringthat the microstructure of the deposited pyrocarbon is constantthroughout the chamber, then it is possible to reduce the gas flow rate.This reduces consumption of carbon precursor, and the size of thepreheating zone can also be reduced without degrading the quality ofpreheating, thereby increasing the working volume of the reactionchamber, and decreasing pumping requirements for maintaining thepressure inside the chamber at the desired value.

The method of the invention can be implemented using annular poroussubstrates other than those shown in FIG. 2, and more generally withsubstrates that are bodies of revolution and that include respectivecentral axial openings or passages.

Thus, in the reaction chamber of the infiltration installation showndiagrammatically in FIG. 3, the porous substrates to be densified arethe fiber preforms for the diverging parts of thruster nozzles. By wayof example, the preforms may be made of carbon fibers that are to bedensified by means of a matrix of pyrolytic carbon.

As shown in FIG. 2, the reaction chamber 41 is cylindrical in shapeabout a vertical axis and is defined by a graphite susceptor 49, agraphite bottom 41a closing the bottom of the chamber, and a graphitecover 41b closing the top of the chamber. The susceptor 49 forms aheater core that is coupled to an inductor (not shown) surrounding thechamber.

The gas containing one or more precursors for carbon is injected intothe chamber 41 via feed ducts 46 leading to the bottom end of thechamber and passing through its bottom 41a. By way of example, the gasmay comprise propane mixed with methane or with natural gas. Theresidual gas is extracted from the top portion of the chamber viaexhaust ducts 47 passing through the cover 41b. The exhaust ducts areconnected to a pumping device (not shown) enabling the desired pressureto be established inside the chamber.

The gas penetrating into the chamber 41 is preheated in a preheatingzone 48 by passing through perforated preheating plates 50 that arespaced apart from one another and from the bottom 41 by spacers 51. Thepreheating plates and the spacers 51 may be made of graphite, forexample. By crossing the preheating zone 48 and passing through theplates 50, the gas is brought to a temperature which is close to thatwhich obtains inside the chamber 41. The preheated gas then passesthrough a diffuser plate 52 having holes 52a and standing on the bottom41a by means of legs.

In this example, there are three preforms to be densified 42a, 42b, and42c which are disposed so that their axes substantially coincide withthe vertical axis of the chamber 41, the diverging parts flaringdownwards. Their flaring or frustoconical shape makes it possible toplace the preforms so that they are partially engaged one withinanother, forming a vertical stack, with the tops of preforms 42a and 42bbeing located inside preforms 42b and 42c respectively.

The preforms are supported by means of respective horizontal trays,namely a bottom tray 45a, and two annular intermediate trays 45b and 45cwhich are spaced apart from one another by spacers 54. A circular topplate rests on the top preform 42a. It has a central opening 58 inalignment with the vertical passage 51 formed by the central channels ofthe preforms being in alignment.

The preheated gas coming from the diffuser plate 52 is channeled towardsa volume 66 situated inside the chamber 41 outside the preforms 42a,42b, and 42c. To this end, the preheated gas is channelled towardscalibrated holes 45 formed through a peripheral zone of the tray 45a,outside the zone on which the downstream end of the preform 42a rests,and it also passes through calibrated holes 45 formed in the peripheralzones of the trays 45b and 45c outside the zones on which the downstreamends of the preforms 42b and 42c rest. The bottom tray 45a may beannular in shape so as to lighten its weight, in which case a ring 53 isplaced between the diffuser plate 52 and the bottom tray 45a so as toprevent the gas gaining access to the passage 61 on leaving thepreheating zone. Under such circumstances, the diffuser plate 52 ispierced only in its peripheral zone.

The intermediate support trays 45b and 45c have respective centralopenings 67 with the walls thereof possibly being substantiallyfrustoconical in shape to match the shape of the outside surfaces of thepreforms they surround, and being of a size that is determined, as isthe height of the spacers 54, so that the trays 45b and 45c co-operatewith the outside surfaces of the preforms 42a and 42b to leave gaps ofpredetermined small width, e.g. one millimeter to a few tenths of amillimeter. A gap of similar width is provided by means of spacers 64between the top of the preform 42c and the top plate 56.

Additional tooling elements may be used, such as annular gaskets 68closing the gaps between the outside edges of the support trays 45a,45b, and 45c, and the inside wall of the susceptor 49, and afrustoconical wall 69 which extends between the inside wall of thesusceptor 49 and the top plate 56, around the outside surface of thepreform 42c so as to define a small volume thereabout. The frustoconicalwall 69 may be fixed beneath the top plate 56.

The supporting trays, the spacers, and other tooling elements usedinside the chamber 41 may be made of graphite, for example.

With the disposition described above, the gas flows from the volume 66outside the stack 60 towards the interior passage 61 from which it isexhausted via the ducts 47. The gaps between the preforms 42a and 42b,and the intermediate support trays 45b and 45c serve to allow pressureto be balanced between the inside and the outside of the stack 60 andalso to allow gas to flow continuously through these gaps, so that theoutside surfaces of the preforms 62a and 62b are immersed in a gas thatis constantly renewed, all the way up to the tops thereof. The gapbetween the top of preform 42c and the top plate 56 also serves tobalance pressure and allows the gas that reaches the top of the volume66 to be exhausted.

In order to optimize retention time, it is preferable to channel the gascoming from the preheating zone into the volume 66 outside the preforms,rather than into the inside volume 61. Unlike the configuration of FIG.2, the volume outside the preforms is smaller than the inside volume,and a greater reduction of retention time is obtained by directing thegas into the smaller of the two volumes. The calibrated orifices 45provide a degree of control over the flow, and the wall 69 contributesto decreasing the volume 66 while still leaving enough space around thepreform 42c.

This case thus likewise produces the above-mentioned advantages of aconsiderable reduction in retention time, particularly with respect toobtaining constant microstructure for the material deposited within thesubstrates along the entire longitudinal direction of the chamber 41,and it also offers the option of reducing the flow rate at which gas isadmitted.

It will be observed that the number of preforms could be other thanthree, depending on the dimensions of the preforms and of theinfiltration chamber, and that is not essential for them to be partiallyengaged one within another, it being possible to use tooling elementsthat can optionally be associated with the support trays to close thespaces between adjacent preforms, while leaving only gaps of smallwidth.

The method of the invention can be implemented with substrates that arenot necessarily annular. It suffices to place the substrates in such amanner as to subdivide the chamber into one or more volumes into whichthe gas can be admitted and one or more volumes from which the residualgas can be exhausted after passing between the substrates or afterdiffusing through them. The substrates can be disposed in one or moreannular or hollow stacks having interior passages defined by thesubstrates.

One such disposition is shown very daigrammatically in FIG. 4. Thesubstrates 70 are in the form of rectangular bars which are stacked insuperposed layers so as to form, in each layer, a polygon that is closedor almost closed, e.g. a triangle. In a stack, the bars 70 thus definean interior volume or passage 80 and an exterior volume 81. Spacers 71are placed between the superposed bars 70 in order to keep them slightlyspaced apart from one another.

The substrates 70 are loaded into a chamber in one or more verticalstacks, e.g. in a manner similar to that shown in FIG. 2. The essentialdifference lies in each annular substrate being replaced by a pluralityof substrates disposed so as to obtain a polygonal shape.

Where appropriate, the internal volume of the reaction chamber can besubdivided into two volumes, with the gas being admitted into one ofthem and being exhausted from the other one, by combining substrates andtooling elements. This may be the case, in particular, when substratesof different shapes and/or sizes are loaded simultaneously.

Although in the preceding examples it is assumed that the preforms aregoing to be densified with pyrolytic carbon, the invention is naturallyapplicable to chemical vapor infiltration using materials other thancarbon, and in particular using ceramics, specifically for manufacturingparts made of ceramic matrix composite material.

In addition, the chamber can be fed with gas and residual gas can beexhausted therefrom respectively from the top portion and from thebottom portion of the reaction chamber, i.e. with the gas flowingdownwards through the chamber, without that bringing into question theprinciples of the invention.

We claim:
 1. A chemical vapor infiltration method for densifying poroussubstrates by depositing material within the substrates, the methodcomprising:loading substrates to be densified into a reaction chamber ofan infiltration oven, with the substrates being disposed in at least oneannular or hollow stack which extends in a longitudinal direction of thechamber and which defines an interior passage; admitting a gascontaining at least a precursor of the material to be deposited into thereaction chamber in the vicinity of a first longitudinal end thereof;channeling the admitted gas towards only one of the two volumesconstituted by the interior passage of the at least one stack and theoutside of the at least one stack, at the end of said stack closer tosaid first longitudinal end of the chamber, with the volume into whichthe gas is channeled being closed at its end farther from said firstlongitudinal end of the chamber; and exhausting residual gas via anoutlet situated in the vicinity of a second longitudinal end of saidchamber opposite from said first longitudinal end; wherein saidsubstrates are stacked while leaving spaces between the substrates whichopen into the interior passage and the outside of said at least onestack to allow pressure to be balanced therebetween, whereby saidchanneled gas is caused to flow from the interior passage to the outsideof said at least one stack, or vice versa, with the gas passing throughthe spaces between the substrates and diffusing into them.
 2. A methodaccording to claim 1, in which the gas admitted into the chamber ispreheated by passing through a preheating zone situated at the first endof the chamber,characterized in that the channeling of the gas towardsthe inside or the outside of the, or each, stack is performed at theoutlet from the preheating zone.
 3. A method according to claim 2,characterized in that;the channelling of the admitted gas is performedtowards the smaller of the two volumes constituted by the inside and theoutside of the stack(s) of substrates; and a compensation element isdisposed inside the, or each, stack, the cross-section of thecompensation element increasing in the flow direction of the gas so asto compensate for lateral leaks through the substrates and between themby reducing the flow section inside the stack in such a manner as tomaintain a substantially constant flow speed for the gas in thelongitudinal direction inside the stack.
 4. A method according to claim1, characterized in that the channelling of the admitted gas isperformed towards the smaller of the two volumes constituted by theinside and the outside of the stack(s) of substrates.
 5. A methodaccording to claim 1 characterized in that a compensation element isdisposed inside the, or each, stack, the cross-section of thecompensation element increasing in the flow direction of the gas so asto compensate for lateral leaks through the substrates and between themby reducing the flow section inside the stack in such a manner as tomaintain a substantially constant flow speed for the gas in thelongitudinal direction inside the stack.
 6. A method according to claim5, for densifying substrates that are substantially in the form of abody of revolution with a central opening andthe substrates are disposedin at least one stack which defines an interior volume formed by thecentral openings of the substrates; the spaces between the preforms inthe, or each, stack are caused to be less than 5 mm thick; the preformsare loaded into the chamber in a plurality of parallel stacks; and thegas admitted into the chamber is channelled towards the interior volumeof the stacks of preforms.
 7. A method according to claim 1, fordensifying substrates that are substantially in the form of a body ofrevolution with a central opening, characterized in that the substratesare disposed in at least one stack which defines an interior volumeformed by the central openings of the substrates.
 8. A method accordingto claim 7, for densifying annular preforms for brake disks,characterized in that the spaces between the preforms in the, or each,stack are caused to be less than 5 mm thick.
 9. A method according toclaim 7, for densifying annular preforms for brake disks, characterizedin that the preforms are loaded into the chamber in a plurality ofparallel stacks.
 10. A method according to claim 9, characterized inthat the gas admitted into the chamber is channelled towards theinterior volume of the stacks of preforms.
 11. A method according toclaim 7, for densifying preforms for the diverging parts of thrusternozzles, characterized in that the gas admitted into the chamber ischannelled towards the volume situated outside the preforms.
 12. Amethod according to claim 7, for densifying preforms for the divergingparts of thruster nozzles, characterized in that the preforms aredisposed one above the other in such a manner that a preform ispartially engaged inside another.
 13. A method according to claim 7, fordensifying preforms for the diverging parts of thruster nozzles,characterized in that the gas admitted into the chamber is channelledtowards the volume situated outside the preforms; andthe preforms aredisposed one above the other in such a manner that a preform ispartially engaged inside another.